1. Field of the Invention
The invention relates to a subminiature electronic device that is mounted directly onto the surface of circuit boards. The invention generally relates to the package of a subminiature device having a sealed cavity over microstructure that is disposed on a substrate. Further, the cavity can be hermetically sealed to enclose a specific gas inside. More particularly, the invention discloses a chip scale packaged device having a hermetic cavity that is fabricated in the substrate level at inexpensive cost.
2. Description of the Prior Art
A hermetic cavity plays an important role in electronic devices by providing a sealed free space over microstructure and enclosing a specific gas inside. Therefore, the microstructure can be operated in a safe and stable environment because air and other gases in the surrounding atmosphere are isolated from the cavity.
The application of hermetic cavities can be roughly classified into four categories. The first category is highly reliable microcircuits and optoelectronics such as the integrated circuits for aviation and military purposes. The hermetic cavity protects electronic circuits from probable corrosion and damage of moisture, oxidation, and mechanical impact. The second category is micro-vibration devices such as oscillators and surface acoustic wave (SAW) filters, which have at least a micro-vibrator on the surface of substrates. A free space must be provided over the micro-vibrator so that the micro-vibrator can be operated functionally. Because the micro-vibrator is very sensitive to moisture and oxidation, a hermetic cavity is required then.
The third category is micro electrical-mechanical devices that usually include swinging or moving elements fabricated from silicon substrates, such as micro-cantilever beams and membranes. It is highly preferred that a hermetic cavity is provided so that the elements can be operated precisely and stably. The fourth category is gas discharge devices such as plasma displays, over-voltage protectors and so on. A hermetic cavity filled with specific discharge gas is required to enable discharge across two separated electrodes at the predetermined voltage.
Polymeric materials are widely used in the electronic industry such as epoxy, polyimide, and silicone, but they cannot hermetically seal the gas. In contrast, metals, glasses, and ceramic materials are able to hermetically seal the gas inside a cavity because of their dense structure. Ceramic materials are suitable for structures such as base, base plate and top cap because of their high melting temperatures. Sealing material usually consists of metals of low-melting temperature or glasses. Tin/gold alloy is a popular metal of low melting temperature that melts at around 350 to 400° C. Glasses melt at somewhat higher temperature, around 400 to 600° C.
To meet miniaturization requirements of final products and systems, electronic devices keep shrinking sizes to be surface-mounted onto circuit boards. FIG. 1A shows a first example of conventional subminiature electronic devices equipped with a hermetic cavity. The example includes a base 13, a chip 11, a lid 18, a cavity 17, an adhesive layer 23, sealing material 28, terminals 22, metallic traces 24, and bonding wires 26.
Base 13 is a rectangular or square body with a cavity that opens on the top surface. The base 13 is made of electrically insulating alumina of more than 90% purity by multi-layer firing at high temperature. Multiple metallic traces 24 are patterned inside the base 13 to connect with corresponding multiple terminals 22 disposed on the bottom surface of the base 13. A metallic film is plated on the top surface of the base 13 to promote adhesion to the sealing material 28.
Chip 11 consists of a substrate and microstructures formed on the top surface of the substrate. The microstructures include micro-electronic circuits, micro-vibration structures, and micro electrical-mechanical structures. Chip 11 is bonded to base 13 by an adhesive layer 23. Bonding wires 26 are extremely tiny metallic wires made of gold or aluminum with a diameter of 25 to 50 microns. Bonding wires 26 electrically connect the microstructures of chip 11 to terminals 22 via the metallic traces 24. Terminals 22 are contact pads when the device is mounted onto circuit boards.
Lid 18 is usually made of metallic plate with gold plating on the surfaces. Sealing material 28 is a metal of low-melting temperature that is usually attached to the lid 18 along its perimeter in a preformed width. Lid 18 together with sealing material 28 is precisely assembled onto the top surface of the base 13 one by one. The entire structure is then placed into a furnace to melt the sealing material 28 and hermetically seal the cavity 17 in the environment of Nitrogen or other specific gases as required.
FIG. 1B illustrates a second example of conventional subminiature electronic device with a hermetic cavity. This example is based on a planar substrate 21 that is topped with microstructures 12. A ceramic cap 19 has a cavity 17 that opens on the bottom surface. Sealing material 25, made of glass with a melting temperature of 400 to 600° C., is applied onto the bottom surface of the cap 19 along its perimeter to hermetically seal the cavity 17. Termination pads 16 electrically connect the microstructures 12 to the end portions of substrate 21. Terminals 14 are contact pads when the device is mounted onto circuit boards.
The device depicted in FIG. 1B can be a chip-type over-voltage protector by gas discharge techniques. Substrate 21 is temperature-resistant and electrically insulating that is normally made of alumina of 96% purity in the rectangular shape. Microstructures 12 are generally two discharge electrodes separated by a tiny gap, wherein one of the discharge electrodes is connected with the circuits to be protected while the other one is connected to the ground. Over-voltage is discharged to the ground across the tiny gap by means of a specific gas enclosed inside the cavity 17.
Two metallic termination pads 16 electrically connect with the two discharge electrodes 12 respectively. The termination pads 16 are usually wider than discharge electrodes 12. The material and thickness of termination pads 16 can be the same as or different from those of the discharge electrodes 12. Two conductive terminals 14 are formed on the side surface of the two opposing ends of the substrate 21 and connect with the discharge electrodes 12 via the two termination pads 16. The terminals 14 are contact pads when the subminiature electronic device is mounted onto circuit boards.
The manufacturing method is to deposit discharge electrodes 12 and termination pads 16 on the top surface of substrate 21. Separately, sealing material 25 is applied onto the bottom surface of the cap 19 along its perimeter. Cap 19 together with sealing material 25 is precisely assembled onto the substrate 21 one by one. The entire structure is then placed into a furnace to melt the sealing material 25 and hermetically seal the cavity 17 in the environment of specific gas. Nowadays, the smallest size of the device is 2.0×1.3 mm (length×width) In the industry according to the structure of this example.
FIG. 1C illustrates a third example of conventional subminiature electronic device with a hermetic cavity, which has the smallest outline in the industry nowadays. It's a chip scale package because the area of the device is just a little bit larger than that of the chip. The device can be a surface acoustic wave (SAW) filter for wireless communication products such as mobile phones that have tight space requirement. The device includes a base plate 15, a chip 11, a cavity 17, metallic traces 24, terminals 22, bonding conductors 30, sealing material 28, and filler 29.
Base plate 15 is generally made of electrically insulating alumina of more than 90% purity by multi-layer firing at high temperature. Multiple metallic traces 24 are patterned inside the base plate 15 to connect with corresponding multiple terminals 22 disposed on the bottom surface of base plate 15.
Chip 11 consists of a piezoelectric substrate and microstructures of vibrators and other microcircuits are formed on the top surface of the substrate. Chip 11 is bonded to the base plate 15 by the flip chip technique so that the top surface faces the base plate 15. Bonding conductors 30, made of gold balls or silver/tin alloy balls, connect the microstructure of chip 11 to terminals 22 via the metallic traces 24 by thermal-sonic means. Therefore, a small gap is defined between the surface of chip 11 and the base plate 15. Sealing material 28, made of a metal of low-melting temperature, is applied onto the surface of chip 11 along its perimeters and melted by heating to hermetically seal the cavity 17.
Terminal 22 are contact pads when the device is mounted onto circuit boards. Filler 29 is polymeric material that covers the bottom surface and side surfaces of the chip 11 to form an outline having same length and width as base plate 15. The smallest size of the device is 1.4×1.1 mm (length×width) in the industry nowadays according to the structure of this example. However, the fabricating cost by flip chip is very expensive because the dimension of chip 11 is extremely small and fine.
The manufacturing method starts from providing a large substrate having disposed multiple base plates 15. Chip 11 is then picked, placed, and bonded to the base plate 15 one by one via bonding conductor 30 by the flip chip technique. Afterwards, the entire structure is placed into a furnace to melt the sealing material 28 and hermetically seal the cavity 17 in the environment of specific gas. The filler 29 is applied to cover the bottom surface and side surfaces of chip 11 to fill up the spaces between two adjacent chips 11. Finally, individual device is separated from the large substrate by dicing of a diamond blade or laser
The described examples of conventional subminiature electronic device with a hermetic cavity have been employed in the industry for many years. But, the material cost is high, particularly the gold-containing material such as sealing material, lid, bonding wires, and bonding conductors. Also, the fabricating cost is high because of the low productivity that the lid and base, or the cap and substrate must be precisely assembled one by one. The fabricating cost is particularly high for a chip scale package by the flip chip technique, wherein the chip is bonded to base plate one by one too. As the device becomes smaller, the conventional assembly technique becomes very challenging to result in much higher fabricating cost.
Bonding a large substrate to another one is a probable solution to reduce the fabricating cost, wherein a large substrate may contain multiple chips, lid, caps, bases or base plates. After the two large substrates complete bonding, individual devices are separated from the large substrate by dicing of a diamond blade, laser or other suitable means. However, bonding two large substrates is very difficult because of thermal mismatch of the two large substrates after melting the sealing material at high temperature unless the material, thickness, and geometry of the two large substrates are exactly the same. Besides, the large substrate containing caps, bases, and base plates is usually made of ceramic material fired at a temperature more than 1000° C. so that the dimensions can not be precisely controlled. The larger the substrate, the more serious the problem will be.
So, the scope of the invention is to provide a subminiature electronic device and the manufacturing method of the same to solve above described problems.